Referring to FIG. 1, there is shown a laser diode 100 and a prior art analog laser diode controller 102. The laser diode 100 has a front facet 110 which emits coherent light that is to be transmitted, usually into some optical component such as an optical fiber 112, and a back facet 114. Light emitted by the back facet 114 is received by a photodiode 116 which is used to continuously monitor the optical power being output by the laser diode 110. In general, the amount of optical power output by the front facet 110 is directly proportional to the optical power output by the back facet 114:
Power(front facet)=Power(back facet) / K
While K is often equal to 1, the amount of back facet power received by the photodiode 116 varies considerably from package to package, and therefore must be separately calibrated for each laser diode.
Typically, the laser diode 100, the back facet photodiode 116, and the outgoing optical fiber 112 (or a mechanism for holding the outgoing optical fiber) are all mounted on a common platform or housing 118. In some cases, the housing 118 includes a solid state thermoelectric cooler for maintaining the laser diode 100 at a specified temperature.
Referring to FIG. 2, the optical output power of the laser diode is a non-linear function of the laser diode's drive current. In particular, when forward bias current is applied to a semiconductor laser it begins to emit light in a manner similar to light emitting diodes (LEDs). This type of emission is known as spontaneous emission because it happens randomly from excited atoms in the laser diode's cavity, and is commonly called LED mode.
At a certain drive current, herein called the threshold current, I.sub.TH, the laser diode's efficiency in converting current into light increases dramatically. This is the point where the laser diode changes from the LED mode of operation to the lasing mode of operation.
While various classes of laser diodes will have thresholds in the same general range of currents, the threshold current I.sub.TH varies considerably among laser diodes of the same type and also varies with the temperature and age of the laser diode. For example, the threshold current of some laser diodes can vary by as much as fifty percent or more with changes in temperature. The effect of this temperature sensitivity is that at a given drive current the laser diode could be operating above its recommended levels at one temperature while not even lasing at another temperature.
When the laser diode is operating in the lasing mode, that is at a drive current in excess of the threshold current, there is a characteristic slope that determines the laser diode's efficiency. More specifically, each laser diode's "slope efficiency" is equal to the ratio of changes in the laser's optical output power to changes in the drive current while operating in the lasing mode. Slope efficiency varies from laser diode to laser diode, and also varies with temperature and with the age of the diode.
The "operating point" or bias current, I.sub.OP, for a diode laser is a generally set by the user of the laser diode so that it is within the current range for lasing mode of operation, and so that the laser diode remains in lasing mode when the current is modulated by an input signal. Thus, if the maximum variation of the input signal below the operation point is MV, the operation point must be greater than I.sub.TH +MV. In addition, the operation point must be set sufficiently high that a receiving photodiode will be able to receive the transmitted light, and yet the operation point must not be set so high as to burn out the laser diode.
Referring back to FIG. 1, prior art diode controllers 102 typically contain an analog feedback loop 120 coupled to a potentiometer 122, or some other similar mechanism, for manually adjusting the laser's operating point. The user typically turns the gain of the feedback loop 120 down before powering on the laser diode's controller, and then manually adjusts the gain upwards until the desired amount of optical output power is achieved. Optical output power is typically measured using another photodiode coupled to the front facet by an optical fiber 112, or by some similar set up (not shown in FIG. 1). After the laser diode's controller 102 is calibrated using potentiometer 112, signals to be transmitted are superimposed on the laser diode's operating point current I.sub.OP by a capacitor 124, thereby modulating the output power of the laser diode 110. Some analog controllers employ multiple potentiometers to separately set threshold current, operating bias current and back facet photodiode feedback control, which components make such analog controllers both difficult to tune and expensive to manufacture.
In general, any laser diode will be destroyed if its optical output power exceeds a certain limit. Given the very sharp slope of the optical output when operation in lasing mode, it is generally quite easy to destroy a laser diode while trying to select its operating point. In fact, many very expensive lasers, such as those used by telephone companies for transmitting telephone signals over optical fibers and those used in the cable television industry, have been destroyed during calibration. Such losses may be caused by adjusting the calibration potentiometer too quickly, by problems in the equipment monitoring the output of the front facet during calibration, causing the laser diode to be turned on too hard, and by many other hazards.
In general, the setup procedure for calibrating laser diodes is time consuming and expensive, and subject to various forms of operator error.
Another important limitation in prior art laser diode controllers is that they cannot be used to predict device failures in advance of when it occurs. Many semiconductor laser diodes are used in vital communications systems, and when such lasers fail, they can cause entire communications systems to fail. If the failure of laser diodes could be accurately predicted, a preventative maintenance program could be implemented to prevent system failures by replacing such laser diodes prior to the time that failure is predicted. Currently, such laser diodes are replaced solely based on their time in service without regard to their actual operability.